Index: trunk/data/validator/geometry.mapcss
===================================================================
--- trunk/data/validator/geometry.mapcss	(revision 6605)
+++ trunk/data/validator/geometry.mapcss	(revision 6611)
@@ -10,2 +10,8 @@
   assertNoMatch: "node bridge=13";
 }
+
+/* Building inside building (spatial test) */
+*[building!~/no|entrance/][coalesce(tag("layer"),"0") = coalesce(parent_tag("layer"),"0")] ∈
+*[building!~/no|entrance/] {
+  throwWarning: tr("Building inside building");
+}